The present invention relates to a method of controlling the rewet shrinkage of a tubular film of non derivatized cellulose to enhance its value as a food casing.
The use of tubular cellulose films as food casings is well known in the art. A non reinforced type of cellulose casing commonly is used in the manufacture of various sausage products such as frankfurters and the like. These cellulose films generally consist of a tubular film of pure regenerated cellulose having a wall thickness ranging from 0.025 to 0.038 mm and ranging in diameters from about 14 to 50 mm.
To date, the most notorious process for manufacturing cellulose food casings is the xe2x80x9cviscose process.xe2x80x9d In the viscose process a natural cellulose is such as cotton linters or wood pulp is converted to a soluble cellulose derivative (cellulose xanthate) by chemical reaction. The soluble cellulose derivative is extruded as a tube and contacted with reagents that react chemically with the cellulose derivative to regenerate a pure form of cellulose.
A more recent innovation for the production of cellulose food casings involves the simple dissolution of the natural cellulose without chemical reaction. In this process the natural cellulose is solubilized by a tertiary amine N-oxide such as N-methyl-morpholine-N-oxide (xe2x80x9cNMMOxe2x80x9d). The solution is thermoplastic in that it is solid at room temperature and has a melting point of about 65xc2x0 C. The solution is extruded as a tube into a bath containing a liquid that is a non solvent for cellulose (such as water) where the solvent is extracted to regenerate a pure form of cellulose. The cellulose derived from this process is xe2x80x9cnon derivatizedxe2x80x9d in that it has not been subjected to covalent bonding with a solvent or reagent as is the xe2x80x9cderivatizedxe2x80x9d cellulose derived from the viscose process. Instead, the non derivatized cellulose has been dissolved by association with a solvent or reagent through Van der Waals forces or by hydrogen bonding.
Regardless of the process used, the regenerated cellulose immediately after tubular extrusion and regeneration is in a gel state. This tubing of cellulose gel is contacted with a plasticizing agent such as an aqueous polyhydric alcohol solution and then is dried under inflation to produce a cellulose film. The gel tubing is initially dried down to a moisture content of less than 10% and preferably 3 to 5% moisture based on the weight of dry cellulose to form a cellulose film. The moisture content based on the weight of dried cellulose is sometimes referred to as the percent of moisture to bone dry gauge or xe2x80x9cBDG.xe2x80x9d Unless otherwise stated, all moisture contents mentioned hereinbelow are on a dry cellulose basis (BDG). Drying the gel tube to a moisture content of 3 to 5% BDG is believed to establish hydrogen bonding in the cellulose structure which is responsible for many end-use properties of the resulting tubular film.
After drying to establish the hydrogen bonding, the tubular cellulose film is remoisturized to a moisture content of about 10 to 20% BDG or higher to provide the film with the suppleness needed for handling and further processing the film without damaging it.
The inflation during the dring process serves to provide the cellulose film with a degree of orientation that also contributes to the properties of the cellulose film. For example, with the conventional derivatized cellulose casing, it is customary to dry the tubing of cellulose gel while holding it by inflation at a diameter that is about 20 to 30% greater than the extrusion diameter to provide transverse direction (TD) orientation. In addition, the tubing is longitudinally stretched by drawing it in the machine direction about 1 to 3% over its length in the gel state to provide a machine direction (MD) orientation.
According to U.S. Pat. No. 2,999,757, the orientation provided by this transverse and longitudinal stretching provides the casing with certain desirable properties including a lower degree of distension upon being subjected to stuffing pressures and the ability to produce a more consistent and uniform diameter of sausage. The ""757 Patent further says that diametrically stretching the casing less than about 35% in the transverse direction does not appreciably improve the casing characteristics whereas stretching it more than about 55% is difficult and presents operating problems. When stretched beyond about 55%, the casing is more apt to break in the dryer. Thus, according to the teachings of the ""757 Patent, the upper limit of transverse orientation for a derivatized cellulose casing is about 55%.
Orienting the film makes it less extensible so that when the casing is stuffed, a change in the stuffing pressure will not result in a substantial change in the stuffed diameter of the sausage. This is a desirable property as it contributes to the production of sausages having a uniform diameter as noted in the ""757 Patent. In particular, it is customary to design a sausage casing to operate at a desired stuffing diameter. When stuffed to this desired or recommended stuffed diameter (RSD), the portion of the curve plotting casing diameter against stuffing pressure is relatively flat so the diameter of the sausage remains substantially constant over a wide range, of stuffing pressures. Sausage manufacturers take advantage of this casing property in that it allows the casing to be tightly stuffed under high pressures of up to about 250 mm of mercury without exceeding a desired diameter.
One drawback of this orientation is that the casing tends to shrink back. to its extruded diameter when it is rewet. The casing is rewet to various degrees when it absorbs moisture during processing or use. For example, the casing is rewet during the shirring of the casing as it as it absorbs moisture from the shirring solution. It also is rewet during stuffing as it absorbs moisture from the fluid products being stuffed into the casing. Excessive shrinkage of the casing during either shirring or stuffing is not desired. Shrinkage during the shirring operation causes the shirred casing to grip tightly about the shirring mandrel. Consequently, damage may result to the casing if excessive force is needed to doff the shirred casing from the mandrel. On the other hand, excessive shrinkage of the casing about the food product stuffed and processed in the casing may cause the casing to split, or the casing may adhere so tightly to the food product that it is difficult to peel the casing from the food product after processing.
Excessive shrinkage is a particular problem with respect to a casing formed of a non derivative cellulose. In this respect it has been found that a non derivatized cellulose casing is much more extensible than its derivatized counterpart. For example, U.S. Pat. No. 5,658,524 discloses that non derivatized cellulose can be diametrically stretched upwards of 200% during the drying cycle. The greater extensibility of the non derivatized casing means that during stuffing, it undergoes a greater diameter change with smaller changes in stuffing pressure than does a comparable derivatized cellulose casing. To compensate for the more extensible nature of the derivatized cellulose casing, it was found necessary to dry the tubing of cellulose gel while holding it under inflation at a diameter of 70 to 80% greater than its extruded diameter. While drying the cellulose gel under these conditions allowed for better control of the stuffed diameter of the casing, the resulting increase in the shrinkage rate upon the casing becoming rewet caused other problems.
For example, because the casing retained a relatively large amount of a shrink (due to its greater degree of orientation), the diameter of the shirring mandrel had to be reduced in order to accommodate the greater amount of shrinkage. This in turn resulted in an undesirably smaller bore diameter of the shirred casing article. Upon stuffing, the casing shrank even more which resulted in the casing being so tight around the sausage in the casing that conventional peeling operations were not satisfactory. Changes in shirring solutions and methods and changes in peeling solutions and methods only marginally solved the various problems caused by the greater inherent extensibility of a casing formed of a non derivatized cellulose.
Accordingly, the problem to be solved is to provide an oriented food casing of a non derivatized cellulose that exhibits resistance to stretching during stuffing so that it maintains a substantially constant diameter over a range of stuffing pressures, while at the same time minimizing the rewet shrinkage of the casing. Prior to the present invention, the method of forming a casing of a non derivatized cellulose represented a compromise of properties in that the method steps for reducing the extensibility of the casing resulted in an increase in the rewet shrinkage. Conversely, method steps to reduce rewet shrinkage resulted in an increase of the extensibility of the casing.
One object of the invention is to provide a food casing formed of a nonderivatized cellulose having a reduced rewet shrinkage.
Another object of the presentinvention is to provide a food casing formed of a non derivatized cellulose that maintains a relatively constant diameter over a range of stuffing pressures.
A further object is to provide a food casing of non derivatized cellulose having both a reduced rewet shrink and reduced diametrical extensibility under stuffing pressures.
Yet another object is to provide an improved method of forming a seamless non derivatized cellulose tubular film suitable for use as a food casing.
A still further object of the present invention is to provide a method of forming a seamless non derivatized cellulose tubular film that has both a reduced extensibility, so its stuffed diameter remains relatively constant with changes in stuffing pressure, and a reduced rewet shrinkage.
The present invention provides a practical method for the manufacture of a seamless tubular film of a non derivatized cellulose that is suitable for use as a food casing for the production of frankfurters and the like. Suitability for use as a food casing requires that a tubular film intended to make a sausage of a desired diameter be dimensionally stable at the desired diameter over a range of stuffing pressures. Suitability further requires that the tubular film exhibit a minimum of shrinkage when rewet. In accordance with the present invention these desirable attributes are provided by controlling the conditions under which the tubular film is dried and moisturized.
Even though it has been recognized that a non derivatized cellulose casing has a greater extensibility and rewet shrinkage than a derivatized cellulose casing, it has been unexpectedly and surprisingly found that properties of dimensional stability and low rewet shrinkage can be built into the tubular film of non derivatized cellulose by controlling certain method steps during the formation of the film. For example, as noted above, an extruded tube of cellulose gel is dried and moisturized to form the cellulose film. It has been found that the inflation of the extruded tube during drying and moisturizing can be altered to change the rewet shrinkage and extensibility of the resulting film.
Accordingly, one embodiment of the invention involves partially drying the tube of cellulose gel while holding it at a first inflated diameter, completing the drying to establish hydrogen bonding of the cellulose while holding it at a second inflated diameter and then moisturizing the cellulose while holding the second diameter.
In another embodiment the extruded tube of cellulose gel is completely dried to establish hydrogen bonding while holding a first inflated diameter and then the dried cellulose is moisturized while holding it at a second inflated diameter.
For use as a food casing where lower values of rewet shrink are desired, the amount of the first inflation is greater than second. This has been found to decrease the total rewet shrinkage as compared to drying and moisturizing under a single inflated condition. While not preferred, if for some reason a high shrink and low extensibility are desired, the method of the present invention allows for such application by reversing the conditions such that the first inflated diameter is less than the second.
Accordingly, in its broadest aspect, the present invention may be characterized by a method of forming a tubular cellulose film of a desired diameter having selected properties of extensibility and rewet shrink comprising:
a) extruding and regenerating a seamless tube composed of a non derivatized cellulose gel;
b) at least partially drying the tube of cellulose gel while holding it at a first inflated diameter different from the desired diameter to form a cellulose film; and
c) moisturizing the cellulose film while holding a second inflated diameter equal to the desired diameter.
In another aspect, the present invention may be characterized by the steps of:
a) drying an extruded tube of non derivatized cellulose gel to a moisture content of less than 5% BDG to effect hydrogen bonding of the cellulose while holding the tube at a first inflated diameter greater than a desired diameter;
b) moisturizing the dried tube to a moisture content of between 10 to 15% BDG while holding the first inflated diameter;
c) collapsing the tube to a flat width;
d) inflating the tube to a second diameter substantially equal to the desired diameter; and
e) moisturizing the tube to a moisture content of between 12 to 25% BDG while holding the second inflated diameter.
In yet another aspect, the present invention may be characterized by:
a) partially drying the tube of cellulose gel to a first moisture content while holding it at a first inflated diameter;
b) collapsing the partially dried tube to a flat width; and
c) inflating the partially dried tube to a second diameter different from the first inflated diameter and maintaining the second inflated diameter while further drying the tube to a second moisture content below about 5% BDG; and
d) moisturizing the dried tube to a moisture content above 10% BDG while maintaining the second inflated diameter thereby forming a tubular to film composed of non derivatized cellulose having shrinkage properties different from the same tube of cellulose gel dried to the same desired diameter and moisture content with only a single inflation.
In still another aspect the present invention may be characterized by a method of forming a seamless non derivatized cellulose tubular film of a desired diameter suitable for use as a sausage casing comprising:
a) continuously forming by extrusion and regeneration, an extruded: seamless tube composed of a hydrated non derivatized cellulose gel having a moisture content greater than 150% BDG, the extruded seamless tube having a substantially uniform diameter smaller than the tubular film desired diameter;
b) inflating the extruded seamless tube to a first diameter larger than the tubular film desired diameter;
c) holding the extruded tube at the first inflated diameter while drying it to a first moisture level of about 15 to 50% BDG thereby forming a partially dried tube;
d) collapsing the partially dried tube to a flat width;
e) reinflating the partially dried tube to a second diameter substantially equal to the tubular film desired diameter;
f) holding the partially dried tube at the second diameter while further drying the tube to a moisture content of less than 5% BDG; and thereafter
g) raising the moisture content of the dried cellulose tube from less than 5% BDG to about 15 to 20% BDG thereby forming a seamless non derivatized cellulose tubular film having enhanced rewet shrinkage properties as compared to a similar cellulose tube that is dried and moisturized while holding at a single inflated diameter.
In a stillfurther aspect, the present invention may be characterized by a tubular film suitable for use as a sausage casing formed by drying a tube of a non derivatized cellulose gel under conditions including partial drying at a first inflated diameter larger than a desired diameter, which preferably is the desired stuffed diameter, followed by further drying and moisturizing at a second inflated diameter substantially equal to the desired diameter, the film having a degree of diametrical dimensional stability over a range of stuffing pressures and a lower rewet shrink as compared to a film formed under the same conditions only with a single drying diameter.
For purposes of using the non derivatized film as a food casing, its rewet shrinkage should be less than about 13.5% and preferably less than 7.5% with shrinkage less than 2 to 3% being most preferred. The extensibility of the tubular film should be relatively flat over a range of pressures so that a change in pressure at which the casing is stuffed results in little or no change in the diameter of the casing. For example over an internal pressure range of 200 to 250 mm of mercury, the extensibility preferably should be less than 1.08. Here the extensibility is D2/D1 where D1 is the diameter at the lower pressure and D2 is the diameter at the higher pressure.
For purposes of using the non derivatized tubular film as a food casing, it has been found that in the course of drying to set the film properties, the film should have a final orientation of greater than 1.5 times the diameter of the extruded tubeand preferably about 1.7 to 1.8 times the diameter of the initial extrusion. Thus, according to the present invention, after regeneration and while the extruded tube is in its gel state (moisture content of 150% or more based on the weight of the cellulose) the first inflation is to a greater diameter or more than 1.7 to 1.8 times the diameter of the extruded tube. The first inflated diameter should be greater than about 1.8 times the diameter of the extruded tube and preferably nearer to 2 times or more the diameter of the extruded tube.
The residence time in the first drying stage (where the greatest inflation occurs) should be sufficient to reduce the moisture content of the extruded tube to about 20 to 50% BDG. It is believed that at this moisture level the cellulose of the partially dried film has not undergone hydrogen bonding. The moisture level of 20 to 50% BDG is believed to be low enough to modify properties and yet leaves the partially dried film sufficiently flexible so it can be collapsed and reinflated to a diameter of 1.7 to 1.8 times the diameter of the extruded film for the second drying stage. In the second drying stage the partially dried tube from the first stage is dried to a moisture level of 5% or less to effect the hydrogen bonding of the cellulose. After drying to this low level to set the film properties, the film is moisturized by passing it through a humid environment to raise its moisture content of about 10 to 12% BDG and preferably higher. This provides the film with the suppleness and flexibility that allows it to be handled for further operations such as winding it onto a reel.
In an alternate embodiment, the moisture content in the first drying stage is driven down to as low as 5% or less while holding the casing at a first inflated diameter to effect the hydrogen bonding of the cellulose. While holding this diameter, the moisture content of the casing is increased to about 10 to 12%. After moisturizing, the casing is collapsed and reinflated to a second diameter. While holding this second diameter, the moisture content of the casing is raised to about 17 to 20% BDG or higher. The effect on the final properties is the same in that a non derivatized cellulose casing made in this fashion has diametrical stability over a range of stuffing pressures at the recommended stuffed diameter while exhibiting a lower rewet shrinkage than a similar casing made using a single inflation during the drying and moisturizing stages.